1. Field of the Invention
The present invention relates to a long wavelength vertical cavity surface emitting laser (VCSEL) with a monolithically grown photodetector.
2. Description of the Related Art
Generally, a VCSEL diode is combined with a photodetector for power monitoring and automatic power control (APC) based on the power monitoring. For example, in U.S. Pat. No. 5,943,357, a photodetector is attached to a long wavelength VCSEL by wafer fusion.
FIG. 1 is a simplified cross-section of a conventional VCSEL to which a photodetector is fused. Referring to FIG. 1, the conventional VCSEL includes an upper semiconductor layer 12 of distributed Bragg reflectors (DBR), an active region 11, and a lower semiconductor layer 13 of DBRs, which are sequentially deposited on a substrate (not shown). The active region 11 is a cavity where laser resonance occurs. A PIN photodetector 20 is fused or bonded to a bottom of the VCSEL having such a configuration.
As described above, a photodetector, for example, a PIN photodetector, is attached to a bottom of a VCSEL with a long wavelength of 1300 to 1600 nm and monitors the power output of the VCSEL. Typically, the attaching technique may be a wafer bonding, a wafer fusion, or a transparent metal adhesion.
Wafer fusion is not suitable for mass-production because of process-related problems. Also, wafer fusion causes a voltage drop at the interface between a photodetector and a VCSEL. As a result, the amount of input voltage must be increased.
A disadvantage of the conventional VCSEL is that a photodetector cannot accurately detect only the output of the VCSEL because both spontaneous emission and a beam emitted from the VCSEL reach the photodetector.
Referring to FIG. 2, both light generated from spontaneous emission and a laser beam emitted from an active region of a conventional VCSEL come out of the active region. Since the structure of the VCSEL is substantially the same as that of a resonant cavity light emitting diode (LED), the spontaneous emission is directed in all directions.
When a VCSEL is designed so that a beam heading for an upper part of the VCSEL can be used as an output, DBRs of a lower semiconductor layer of the VCSEL have higher refractive indices than those of an upper semiconductor layer thereof. Accordingly, the intensity of a laser beam heading for a lower part of the VCSEL is relatively lower than that of the beam heading for the upper part of the VCSEL. Because a laser beam emitted from a VCSEL typically has a diameter of about 10 μm but spontaneous emission is directed in all direction, the intensity of the laser beam is higher than that of the spontaneous emission at a specific area of a photodetector where the laser beam passes (i.e., at an area with an approximately 10 μm diameter located directly down a center of the VCSEL). However, the percentage of spontaneous emission received by the entire area of the photodetector is quite high. Particularly, this feature appears in the VCSEL shown in FIGS. 1 and 2, to a bottom surface of which a light-receiving surface with an about 200-300 μm width of the photodetector 20 is bonded so as to receive light from the VCSEL.
In a short wavelength VCSEL with a monolithically-grown photodetector, the photodetector must be attached to an upper surface of the short wavelength VCSEL because, if the photodetector is attached to a bottom surface of the VCSEL, a substrate corresponding to the base of the VCSEL absorbs a laser beam emitted from the VCSEL and accordingly cannot monitor the exact power output of the VCSEL.